doc: improve path.posix.normalize docs

Add section to path docs that explains that path.posix.normalize
does not replace Windows slashes with POSIX slashes because POSIX
does not recognize / as a valid path separator.

Fixes: https://github.com/nodejs/node/issues/12298
PR-URL: https://github.com/nodejs/node/pull/12700


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Cai <davidcai1993@yahoo.com>